This invention relates to an improved method and apparatus for the precise administration of parenteral fluids whereby a clamp is repeatedly opened and closed to yield an exact, predetermined volumetric flow rate under an extremely wide range of operating conditions.
The common method of intravenous fluid administration involves connecting an inverted flask of nutrient fluid to a closed tubing system which transports the fluid to a needle or cannula within the venous system of a patient. The flow of liquid is maintained by the force of gravity operating on the elevated liquid column. The flow rate is controlled by a manually operated clamp which changes the internal cross-sectional area of the tubing. Flow rates are measured by means of a drip chamber located at the upper end of the tubing system. The fluid from the flask passes through a drop former in the drip chamber. The drip chamber serves the dual function of allowing a nurse or other attendant to observe the rate at which the liquid drips out of the bottle, and also creates a reservoir for the liquid at the lower end of the chamber to ensure that no air enters the main feeding tube leading to the patient. For most administrations the drop formers used produce approximately 15 drops per milliliter. When slower administration of fluid is desired, drop formers producing approximately 60 drops per milliliter are used. It is general practice in most hospitals for the nurse to periodically monitor flow rates by counting the number of drops within a specific time interval. The nurse will mentally perform the mathematics necessary to convert the timed drop count to an appropriate rate, e.g. in cubic centimeters per hour.
Precise regulation of the flow rate requires time consuming clamping adjustments when the intravenous system is set up by the nurse. Relatively constant surveillance is required to maintain a stable flow rate due to physiological fluctuations, changes in the mechanical conditions of the intravenous system, or to exhaustion of liquid supply when the flask is empty. In most hospital situations, continuous monitoring of intravenous systems is neither practical nor economically feasible. Periodic monitoring is prone to error and may seriously compromise patient safety and the effectiveness of medical therapy. Accurately controlled flow rates are needed in a number of situations, such as with eldery, cardiac, obstetrics and pediatric patients in which precise fluid volumes or pharmacological doses are to be infused. This is increasingly important because intravenous therapy is becoming a more routine procedure.
In recent years a number of electronic monitoring systems, drop flow controllers and infusion pumps have been developed to accomplish the various tasks of sensing and regulating drop flow rates. The automatic flow controllers as proposed in the past basically compare a reference drop rate to the actual drop rate. This comparison yields an error signal used to control some kind of flow adjustment apparatus. Various methods of detecting drop frequency have been proposed utilizing optical, thermal, mechanical, conductive, electromagnetic or capacitive means. Some of the fluid control means have included electromagnetically operated pressure clamps and check valves. All of these systems have not always proved to be entirely satisfactory because they are feedback control systems in which the transfer function of the system is not well known, or indeed, constant in time. Consequently the response time of such systems cannot be very rapid, and over short periods of time flow rates can be significantly different from those desired. In most of these systems no account is taken of the variation of drop size. Variations in the patient condition such as fluctuations in intravascular pressure are not readily determined by the current monitoring methods.
Typical monitoring systems are shown in U.S. Pat. Nos. 3,890,968; 3,756,556; 3,736,930; 3,790,042; 3,826,137; 3,469,574 and 3,800,794.